Which component is not used to vary the flow of refrigerant in a refrigeration system?

The solenoid valve is not responsible for varying the flow of refrigerant in a refrigeration system. Instead, its primary function is to control the on/off state of a circuit. In refrigeration systems, varied flow of refrigerant is accomplished by components such as the expansion valve, which regulates the amount of refrigerant entering the evaporator, and the compressor, which compresses the refrigerant to facilitate its movement through the system. The condenser also plays a crucial role in heat exchange and phase change but does not directly control flow rates.

By focusing on the functions of these components, it's clear that the solenoid valve's role is more about controlling the system rather than managing refrigerant flow itself, making it the correct choice for this question.